Addressing
Formats and QoS parameters:
:
When an application process
wishes to setup a connection
to a remote application process,
it must specify which one to
connect to. The method normally
used is to define transport
addresses to which processes
can listen for connection requests.
In the internet, these end points
are pairs. In ATM networks,
they are AAL-SAPs, we eill use
the neutral term TSAP (Transport
Service Access Point). The analogous
end porints in the network layer
(ie., network layer addresses)
are then called NSAPs. IP addresses
are examples of NSAPs.
The format of a transport protocal
data unit (TPDU) is shown below.
Each TPDU consists of four general
fields : length, fixed parameters,
variable parameters and data.
| Length |
Fixed parameters |
Variable parameters |
Data |
Length : The length field
occupies the first byte and
indicates the total number of
bytes (excluding the length
field itself) in the TPDU.
Fixed Parameters : The fixed
parameters field contains parameters,
or control fields that are commonly
present in all transport layer
packets. It consists of five
parts: Code, source reference,
destination reference, sequence
number, and credit allocation.
Code : The code identifies
the type of the data unit for
example, CR for connection request
or DT for data.
CR : Connection request
CC : Connection confirm
DR : Disconnect request
DC : Disconnect Confirm
DT : Data
ED : Expedited data
AK: Data acknowledge
EA : Expedited data acknowledge
RJ : Reject
ER : Error
Source and destination reference
: The source and destination
reference fields contain the
addresses of the original sender
and the ultimate destination
of the packet.
Sequence Number : As a transmission
is divided into smaller packets
for transport, each segment
is given a number that identifies
its place in the sequence. Sequence
numbers are used for acknowledgement,
flow control, and reordering
of packets at the destination.
Credit Allocation : Credit
allocation enables a receiving
station to tell the sender how
many more data units may be
sent before the sender must
wait for an acknowledgement.
Variable parameters
: It contains parameters
that occur in frequently. These
control codes are used mostly
for management.
Data : It
may contain regular data or
expedited data coming from the
upper layers. Expedited data
consist of a high priority message
that must be handled out of
sequence. An urgent request
can supersede the incoming queue
of the receiver and be processed
ahead of packets that have been
received before it.
QoS : Quality of Service
:
Another way of looking at the
transport layer is to regard
its primary function as enhancing
the QoS (Quality of Service)
provided by the network layer.
If the network service is impeccable,
the transport layer has an easy
job. If however, the network
service is poor, the transport
layer has to bridge the gap
between what the transport users
want and what the network layer
provides.
The transport service may allow
the user to specify preferred,
acceptable, and minimum values
for various service parameters
at the time a connection is
setup.
Now we will discuss
some of the QoS parameters
The connection establishment
delay is the amount of time
elapsing between a transport
connection being requested and
the confirmation being received
by the user of the transport
service. It includes the processing
delay in the remote transport
entity. As with all parameters
measuring a delay, the shorter
the delay, the better the service.
The connection establishment
failure probability is the chance
of a connection not being established
with in the maximum establishment
delay time, for example, due
to network congestion, lack
of table space some where, or
other internal problems.
The Throughput parameter measures
the number of bytes of user
data transferred per second,
measured over some time interval.
The throughput is measured separately
for each direction.
The Transit delay measures
the time between a message being
sent by the transport user on
the source machine and its being
received by the transport user
on the destination machine.
As with throughput, each direction
is handled separately.
The Residual error ratio measures
the number of lost or garbled
messages as a fraction of the
total sent. In theory, the residual
error rate should be zero, since
it is the job of the transport
layer to hide all network layer
errors. In practice, it may
have some finite value.
The Protection parameter provides
a way for the transport user
to specify interest in having
the transport layer provide
protection against unauthorized
third parties (wire tapers)
reading or modifying the transmitted
data.
The priority parameter provides
a way for a transport user to
indicate that some of its connections
are more important than other
ones. And in the event of congestion,
to make sure that the high-priority
connections get serviced before
the low-priority ones.
Finally, the Resilience parameter
gives the probability of the
transport layer itself spontaneously
terminating a connection due
to internal problems or congestion.
The QoS parameters are specifies
by the transport user when a
connection is requested. Both
the desired and minimum acceptable
values can be given . In some
cases, upon seeing the QoS parameters,
the transport layer may immediately
realize that some of them are
unachievable.
